Volume Bragg gratings (VBG) are diffractive optical elements. They are typically holographically recorded by exposing a photo-sensitive material to a two-beam interference pattern, creating a spatial sinusoidal refractive index modulation. An incident wave that satisfies the Bragg condition will be diffracted at an angle determined by a grating vector and a wave vector of an incident beam. Detuning from the Bragg condition in either angle or wavelength will reduce the diffraction efficiency, meaning the element typically has narrow spectral and angular selectivity. In the past, these features have been useful for spectral beam combining, spectral narrowing of laser output, spectral filtering, and transverse mode selection in laser resonators.
Another important feature of these gratings is that several holographic elements can be recorded within the photosensitive material, forming a multiplexed volume Bragg grating (MVBG). By designing such a holographic element to diffract two incident beams of a specific wavelength, which are incident at different angles, along a shared path, each grating will interact with one another, allowing multiple incident wavefronts to coherently interact.
In the past, these multiplexed gratings have been used as elements to coherently combine multiple outputs from separate laser resonators, allowing for the mode area to be increased by a factor equal to the number of gratings recorded in the photosensitive material.
An example of a recording medium for VBGs is photo-thermo-refractive (PTR) glass, which is a photosensitive glass with a high laser damage threshold, low absorption, and wide transparency region, making it a suitable substrate for high power systems. This glass is photosensitive in the near UV spectral region and it is transparent from 350 to 2700 nm. Recording of MVBGs has previously been demonstrated, allowing for the design and construction of coherently combined fiber lasers.
Recording VBGs in PTR glass has enabled the design of narrow band spectral and angular filters. The use of angular filters as a transverse mode selecting element has been demonstrated in several types of resonators. In both solid state and fiber lasers, single transmission VBGs have been used to angularly filter the higher order modes, allowing only the fundamental mode to oscillate in the resonator, improving the beam quality and brightness of the laser. In diode lasers, a tilted VBG was used to provide feedback for one of the lobes of a higher order mode. For this system, only half of the two lobed far field profile is diffracted providing feedback while the second lobe is transmitted as the output power. Although a higher order mode oscillates within the resonator, a single, diffraction limited beam is output from the system.
Creation of MVBGs, and their use to coherently combine several lasers, has been demonstrated in several publications. In these systems, multiple laser sources interfere on a single optical element, and depending on the relative phasing between the sources can constructively interfere to produce a single diffraction limited beam. Each laser source is in phase with each other, and N gratings are required to combine N laser sources, increasing the mode area by N times.
Previous methods of mode conversion depend on the use of binary phase plates, which consist of a number of π phase discontinuities proportional to the mode number being converted to. Such an element, when placed in the near field image of the mode profile, will either correct the it phase discontinuities of the higher order mode to convert it to a fundamental mode, or will add a number phase discontinuities to a fundamental mode to convert it to a higher order mode. However, such elements: (1) have the distinct disadvantage of not providing angular selectivity, meaning they cannot be used as an angular filter in a resonator and (2) can be difficult to manufacture due to the strict requirements on the slope of the π phase discontinuity.
Previous methods of transverse mode selection using angularly selective elements force the fundamental mode to oscillate within the gain medium, improving beam divergence while reducing the mode area. A previous method of higher order mode selection using a tilted VBG could select a single higher order mode, but did not provide the mode conversion outside of the gain medium.
Previous applications of a MVBG to coherently combine several laser systems required N gratings to be recorded in a single optical element to increase the mode area N times. This requirement places serious constraints on the minimum thickness of the MVBG, increasing heating when used in a high power system.
The inventors have recognized the benefits and advantages of systems and methods enabling mode selection and conversion that would allow the mode area of a single laser to be increased without requiring more than two gratings to be recorded within a single optical element. It would be particularly advantageous to allow the mode area to increase as a function of the angular selectivity of each grating and the angular difference between the incident wavevectors, allowing the mode area to increase independently of the number of gratings recorded in the MVBG. Mode conversion with a MVBG has the advantage of either providing both mode selection and mode conversion in a resonator, or mode conversion in a passive system.
These and other objects, benefits, and advantages provided by the solutions enabled by the embodied invention will be described in detail below with reference to the accompanying figures and as set forth in the appended claims.